Interest in catalysis continues to grow in the polyolefin industry. Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. To improve polymer properties, single-site catalysts, in particular metallocenes are beginning to replace Ziegler-Natta catalysts. Single-site catalysts typically require large amounts of expensive activators such as methylalumoxane or salts of non-nucleophilic anions such as triphenylcarbenium tetrakis(pentafluorophenyl)borate. It would be desirable to improve polyolefin properties without the high cost of single-site catalysts and their activators.
Ziegler-Natta catalyst systems are well known in the art. Useful Ziegler-Natta catalysts include titanium compounds and their combinations with aluminum compounds. It is known to support the titanium compound with compounds such as silica or magnesium chloride and considerable research has been done in this area. Known compositions also include an aluminum compound, sometimes referred to as a cocatalyst. Trialkyl aluminums, dialkyl aluminum halides, and alkyl aluminum dihalides are common cocatalysts.
It is known to add other compounds to a Ziegler-Natta catalyst system to influence catalytic properties. Various Lewis bases have been used; they are often referred to as modifiers or electron donors. When the electron donor is added during the preparation of the Ziegler-Natta catalyst system it is sometimes called an “internal donor,” while those added during or immediately prior to the polymerization have been called “external donors.” A variety of electron donors have been disclosed (for example, see U.S. Pat. No. 4,136,243). Common electron donors include ethers and esters (for example, see U.S. Pat. No. 5,968,865), but many others have been used. U.S. Pat. No. 5,106,926 gives examples of suitable electron donors as alkyl esters of aliphatic or aromatic carboxylic acids, aliphatic ketones, aliphatic amines, aliphatic alcohols, alkyl or cycloalkyl ethers, and mixtures thereof with tetrahydrofuran being preferred. U.S. Pat. No. 4,927,797 discloses the use of silane donors such as methylcyclohexyldimethoxysilane, and U.S. Pat. No. 6,228,792 discloses the use of 2,6-disubstituted pyridines as electron donors. Sometimes two or more electron donors are used. U.S. Pat. No. 7,560,521 teaches a combination of a monofunctional donor selected from ethers, esters, amines, or ketones with a difunctional donor selected from diesters, diketones, diamines, or diethers. U.S. Pat. No. 6,436,864 discloses unsaturated nitrogenous compounds as electron donors. An imine, a diimine, and a methoxymethylpyridine are used in the examples. A nitroso compound is not disclosed.
Chelating N-oxide ligands have been used in constructing single-site catalysts. For example, U.S. Pat. Nos. 6,498,221 and 6,875,829 use chelating N-oxides such as 2-hydroxypyridine N-oxide to synthesize certain single-site catalysts. They are not used with Ziegler-Natta catalyst systems. Nitroso compounds are not used. U.S. Pat. Nos. 4,168,358 and 6,541,592 disclose the use of pyridine N-oxide as an alternative to hydrogen or diethyl zinc to regulate molecular weight in a vanadium-based Ziegler-Natta polymerization.
U.S. Pat. No. 3,444,149 uses nitroso compounds to lower the molecular weight of polymers prepared with vanadium-based Ziegler-Natta catalyst systems. They disclose that the molecular weight may be remarkably reduced and that liquid polymers may be obtained in this way.
The role of donors is not completely understood and remains a subject of continued research. As polyolefin applications become more demanding, there is a continued need for improvements in catalyst systems. Despite the considerable research that has been done in this area, apparently no one has studied nitroso compounds as a component in a titanium-based Ziegler-Natta catalyst system or contemplated that they might be used to increase polyolefin molecular weight.